Alkaline storage batteries are excellent in overcharge resistance and overdischarge resistance and are batteries easy to use for general users. Because of this, alkaline storage batteries are in extensive use as the powder sources of portable telephones, small power tools, and portable small electronic appliances such as personal computers. The demand therefor is remarkably growing with the spread of these small electronic appliances. Alkaline storage batteries have been put to practical use also as the driving power sources of hybrid electric vehicles (HEV).
In the case of the alkaline storage batteries heretofore in use, 100% charging of a battery in which the capacity has been used up necessitates at least 1 hour. If a reduction in the time required for charging can be attained, convenience for users is improved. There is hence a desire for the development of a technique of high-rate charging, which attains a further reduction in the time required for charging, besides an improvement in discharge capacity.
Factors which inhibit high-rate charging are as follows. In rapid charging, the battery temperature rises due to the heat of reaction and Joul's heat and materials constituting the battery alter to deteriorate battery characteristics. High-rate charging, for example, accelerates the deterioration of the hydrogen-storing alloy.
Furthermore, the internal pressure of the battery increases during charge, and this may result in the leakage of a gas, which is a product of the decomposition of the liquid electrolyte, or of the liquid electrolyte. Because of this, repetitions of high-rate charging accelerate the consumption of the liquid electrolyte as compared with ordinary charging, resulting in a possibility that the cycle life might be reduced.
A pulse charging technique such as that described in, e.g., JP-B-47-45462 (page 7, claims) has been proposed as a technique for enabling rapid charging. However, even when the pulse charging technique is applied to the charging of alkaline storage batteries, it has been impossible to complete the charging in a time period as short as about a half of an hour or shorter.
In WO 02/35618 A1 (FIG. 2A) are proposed a charging method and apparatus therefor in which a sealed battery having a pressure switch (pressure response switch) function is charged in such a manner that when the internal pressure of the battery exceeds a specified value during charge, the charging is stopped and when the internal pressure of the battery is not higher than the specified value, charging is conducted.
However, the method proposed in that patent document has had drawbacks that the charging efficiency is low in short-time charging in up to 30 minutes and that when the method is applied to a sealed storage battery having an increased capacity, the drawback of low charging efficiency becomes conspicuous.
The nickel/metal-hydride battery and nickel-cadmium battery mentioned above each employ a nickel electrode as the positive electrode. This nickel electrode is one obtained by impregnating a porous nickel substrate such as foamed nickel with a paste containing an active-material powder comprising nickel hydroxide as the main component. The negative electrode (hydrogen-storing-alloy electrode) of the nickel/metal-hydride battery is one obtained by adding a thickener and a binder to a hydrogen-storing-alloy powder to prepare a paste and filling this paste on a substrate such as, e.g., a punching metal foam formed from a nickel-plated steel sheet.
The negative electrode (cadmium electrode) of the nickel-cadmium battery is one obtained in the same manner as for the hydrogen-storing-alloy electrode, except that a powder comprising cadmium oxide or cadmium hydroxide as the main component is used in place of the hydrogen-storing alloy.
When alkaline storage batteries are charged, oxygen generates at the positive electrode in a final stage of charge. In the alkaline storage batteries, the oxygen which has generated at the positive electrode is absorbed by the negative electrode, whereby the batteries can be produced in a sealed form.
In the alkaline storage batteries heretofore in use, the proportion of the impregnant capacity of the negative electrode to the impregnant capacity of the positive electrode has been set at from 1.5 to 1.8 or higher (large-excess negative-electrode impregnation) so as to accelerate oxygen absorption by the negative electrode during charge and to inhibit hydrogen generation at the negative electrode.
In case where the proportion of the impregnant capacity of the negative electrode to the impregnant capacity of the positive electrode in the alkaline storage batteries heretofore in use is regulated to below 1.5, a charge reserve cannot be sufficiently secured and there is a possibility that the amount of hydrogen generating at the negative electrode in a final stage of charge might increase to heighten the internal pressure of the battery.
Furthermore, there is a possibility that during repetitions of charge/discharge, γ-NiOOH, which is inactive as an active material, might generate and accumulate on the positive electrode to reduce the capacity, or that the oxygen which has generated at the positive electrode might corrodes the hydrogen-storing alloy or cadmium to reduce the capacity.
In producing the nickel electrode, cobalt monoxide or cobalt hydroxide is added, besides the nickel hydroxide powder as an active material, in order to enhance electrical conduction in the electrode. After incorporation into a battery, this electrode is charged to thereby oxidize the cobalt hydroxide to a conductive higher-order compound (also called cobalt oxyhydroxide).
The, reaction which thus yields a higher-order cobalt compound through charging is an irreversible reaction. Consequently, in the case where a higher-order cobalt compound has been yielded by charging, it is necessary that latent electricity should be stored in the negative electrode as a discharge reserve in an amount corresponding to the quantity of electricity consumed by the formation of the higher-order cobalt compound and the quantity of electricity to be consumed for oxidizing the cobalt added to the nickel hydroxide so as to form a solid solution. The charge reserve decreases accordingly. A decrease in charge reserve amount has caused a possibility that the internal pressure of the battery might increase during charge or the hydrogen-storing alloy or cadmium as a negative-electrode active material might be corroded, leading to a decrease in charge/discharge cycle life.
For securing a necessary charge reserve amount, it is necessary in designing a battery to estimate the impregnation with an active material for forming a discharge reserve in the negative electrode.
Such additional impregnation with a negative-electrode active material based on the estimation for discharge reserve formation further reduces the amount of the positive-electrode active material used for impregnation, resulting in a decrease in the discharge capacity of the battery.
For example, JP-A-3-78965 (page 3, left upper column, lines 14-16) and JP-A-4-26058 (page 2, right upper column, lines 9-10) propose a method in which a cobalt compound, such as, e.g., cobalt hydroxide, deposited on the surface of nickel hydroxide as an active material for nickel electrodes is oxidized to a higher-order cobalt compound beforehand in a chemical manner in order to inhibit the formation of a discharge reserve.
However, even when this method is used, it is still necessary to regulate the negative-electrode capacity/positive-electrode capacity ratio to at least 1.5-1.7 and it has been difficult to reduce this proportion to a smaller value. Consequently, the alkaline storage batteries heretofore in use have had a drawback that the capacity of the positive electrode must be reduced in order to pack the negative electrode in large excess into a predetermined battery volume and, hence, the battery capacity is limited to low values.
For inhibiting oxygen generation at the positive electrode during charge, a method in which a compound of a rare-earth element is added to a nickel electrode is proposed in, e.g., JP-A-9-265981 (page 2, right column, lines 33-38).
The addition of a compound of a rare-earth element to a nickel electrode is effective in shifting the oxygen evolution potential of the nickel electrode to the noble side. Because of this, the difference between the oxygen evolution potential and the potential of the nickel electrode enlarges and oxygen evolution is inhibited, resulting in an improved charging efficiency.
However, even with the expedient described above, high-rate charging results in enhanced oxygen generation at the positive electrode and, hence, the rate of oxygen absorption at the negative electrode is too low as compared with the rate of oxygen generation at the positive electrode. In addition, since hydrogen generation at the negative electrode occurs simultaneously, there is a possibility that the internal pressure of the battery might increase abruptly.
Because of these, the rate of charging in the alkaline storage batteries heretofore in use has been limited to 1-hour-rate charge (1-ItA charge), and charging at a higher rate has been difficult.
On the other hand, small storage batteries such as, e.g., cylindrical storage batteries employ an electrode structure comprising a-rectangular electrode and a tab-form lug (hereinafter referred to simply as lug) bonded thereto, and employ an element obtained by spirally winding an assembly composed of such electrode structures and separators stacked therewith.
In storage batteries heretofore in use, the lug in an electrode structure has been bonded in the position shown in FIG. 10 in order to prevent the lug from coming into contact with the metallic battery case to cause internal short-circuiting. Namely, the lug 23 in the electrode structure 21 is bonded in such a position that the distance b between that shorter side 22a of the rectangular electrode which faces the center and the center line X of the lug 23 satisfies the following relationship with the length a of a longer side of the electrode: b≦0.4a.
As long as charging is completed in about 1 hour or longer as before, use of that electrode structure has posed no problem. However, it has been found that when the rapid charging which has come to be newly desired, in which charging is completed in a period as short as 15-30 minutes, is attempted, then a high charging efficiency cannot be obtained and an abnormal battery temperature increase occurs to cause a decrease in capacity or abrupt deterioration of cycle characteristics.
A subject for the invention, which has been achieved in view of the drawbacks of related-art techniques described above, is to provide an alkaline storage battery which employs a hydrogen-storing-alloy electrode or cadmium electrode as a negative electrode and which has a high discharge capacity and has a high charging efficiency even in charging in an extremely short time, which has not been attained so far, without undergoing a decrease in the coefficient of active-material use or a decrease in charge/discharge cycle performance or in the function of inhibiting the internal battery pressure from increasing during overcharge or high-rate charging. Another subject is to provide a method of charging the battery.
The invention has been achieved in view of the problems of related-art techniques described above. A further object thereof is to provide an electrode structure and a storage battery which each can attain improvement in suitability for rapid charging completed in 15-30 minutes, without lowering battery characteristics concerning smallness, high capacity, and cycle performance. It was found that the electrode structure produces surprising effects due to the lug bonding position therein which has been optimized, although this has not been attained with any technical idea in the related art. Those objects have been thus accomplished.